If you are lucky, then visio import filter might help you: http://ask.libreoffice.org/question/44/visio-import-filter

About websites being "reputable" or not, I have not heard of any cases where opening file with OpenOffice or LibreOffice with Macros disabled was harmful. Just make sure it is not .exe, .com or .bat file or use open dialog in LibreOffice. After opening save the file from LibreOffice, and you should be all good.